Spinal CT scans, plain radiographs, and medical records of 81 patients with an abnormal pars interarticularis were reviewed to define the CT criteria for spondylolysis. In many cases, it was difficult to detect because it simulated the adjacent facet joints; however, it could be differentiated by careful analysis of the section level and the contiguous facet joints, which usually had regular cortical surfaces (in contrast to the pars defects). Spondylolysis was evident on the lateral localizer image in most cases. In some patients the pars appeared abnormally narrow and elongated or sclerotic as well as interrupted. In a few cases, CT demonstrated a pars defect which was not effectively shown by the plain radiographs. In one patient, a herniated disk was also seen.